1. Field of the Invention
The present invention relates to a method of transmitting feedback information, and more particularly, to a method of transmitting feedback information in a Multi-Input, Multi-Output (MIMO) system.
2. Discussion of the Related Art
A Multi-Input, Multi-Output (MIMO) system seeks to improve frequency efficiency and capacity of a network link by using a plurality of antennas in a base station (BS) as well as in a mobile station (MS). As such, the MIMO system has been receiving much attention as useful technology for transmitting high speed data in a mobile wireless communication system.
In a closed-loop MIMO system having plurality of antennas, the MS receives data transmitted from the BS and estimates channel information of each channel. Then the MS uses the estimated channel information to obtain a weight value which is subsequently transmitted as feedback information to the BS. After receiving the feedback information, the BS applies the weight value to input data so as to more effectively and efficiently transmit the input data to the MS.
In an Orthogonal Frequency Division Multiple/Orthogonal Frequency Division Multiple Access (OFDM/OFDMA) system, the BS receives from the MS various types of feedback information to be applied to downlink signals. In the MIMO system, there are various modes that can be applied for transmission based on the data transmission scheme used by the multiple antennas of the transmitting end. These modes include a Space Time Transmit Diversity (STTD), a Spatial Multiplexing (SM), and a closed-loop SM. Furthermore, in the closed-loop MIMO system, there are five (5) types of feedback information that can be transmitted to the BS from the MS.
The first type is an antenna grouping index. The antenna grouping index applies when the closed-loop STTD scheme is used. More specifically, a pair of antennas, which has achieved STTD, can be changed based on channel status, and an index of the optimum pair of antennas are provided by the MS to the BS.
The second type is an antenna selection index. The antenna selection index applies when a closed-loop STTD scheme or a SM scheme is used. More specifically, a specified number of antennas having best channel status are selected from a plurality of antennas, and indices of the selected antennas are sent to the BS.
The third type is a precoding codebook index. The precoding codebook index applies when a closed-loop SM scheme is used. More specifically, the precoding matrix codebook, which is multiplied to the transmission signal at the BS transmission antenna group, are retained by both the BS and the MS, and the MS selects and provides ith the optimum precoding matrix with respect to channel status to the BS.
The fourth type is a directed precoding matrix value. The directed precoding matrix value applies when a closed-loop SM scheme is used. More specifically, the MS directly provides a value of the precoding matrix. The precoding matrix is multiplied to the transmission signal at the BS transmission antenna group.
The fifth type is a number of streams. The number of streams applies when a STTD scheme or a SM scheme is used. More specifically, the MS provides a number of transmission signals, which can be used to achieve optimum transmission capacity in either scheme, to the BS. Here, the number of transmission signals is identical to the number of simultaneously transmitted transmission symbols.
In addition to five (5) types of feedback information, the feedback information can include the MS selected MIMO mode a permutation scheme associated with the selected mode. The details of which are provided below.
In the OFDM/OFDMA wireless communication system, the MS determines the permutation scheme and transmits the determined permutation scheme to the BS in a form of feedback information. Here, determining permutation scheme means that the MS determines the scheme by which data in a frequency broadband for data transmission is allocated and arranged. That is, the MS selects any one of a Full Usage Sub-Carrier (FUSC), a Partial Usage Sub-Carrier (PUSC), and an Adjacent Sub-Carrier Permutation schemes.
Transmitting the feedback information of the MIMO mode and the permutation scheme selected by the MS or the weight to be applied for downlink transmission in the closed-loop MIMO system can be accomplished using the following processes. First, the BS transmits to the MS information containing the feedback types to be used in sending feedback information to the BS and the feedback time period according which the MS should transmit feedback information to the BS. This information can be transmitted using any one of an Information Element (IE) for providing a Channel Quality Information Channel (CQICH), a fast feedback allocation subheader which is transmitted together with data, and a Feedback Polling IE which is used by the BS to allocate uplink resource.
After receiving the information the feedback types and the feedback period, the MS can use a fast feedback channel on an allocated frame to send feedback information periodically or use a feedback header in the allocated uplink resource to transmit feedback information periodically or non-periodically. If there is data to be transmitted to the BS, the MS uses the subheader to transmit the data.
The following problems exist with respect to the above description. First, a procedure for transmitting (reporting) a MIMO mode and permutation scheme selected by the MS is different from a procedure for transmitting (reporting) determined weight. Second, although there are currently three (3) types of methods for transmitting weights to be applied to downlink transmission, there is no information as to which method should be applied. Lastly, there is no disclosure relating to a method of transmitting a direct precoding matrix value.